In greenhouse plant production systems, seedlings are germinated in high density trays to effectively utilize the limited floor space in the greenhouse. Once the seedlings have achieved a certain level of maturity, they are transplanted into low density flats for further growth and development. In plug production systems, seedlings are normally germinated in plug trays having a large number of plug cells per unit area, for example, 20 cells per row and 30 rows on a tray measuring 334 mm. .times. 492 mm. .times. 12.7 mm. deep. After a certain growing period, the seedling plugs are then transplanted into growing flats at a lower density. For example, eight rows with six seedlings per row may be transplanted into a flat which is 350 mm. .times. 450 mm. .times. 50.8 mm. deep. There may be many variations in the dimensions of the plug trays and growing flats and in the density in which the seedlings are arranged.
Currently, the transplanting operation in most instances is a manual labor intensive task. In order to avoid the need for expensive manual labor in the greenhouse, various robotic systems have been investigated for automatically transplanting seedlings from high density plug trays to low density growing flats in an effort to reduce the cost of operating a greenhouse. Those robotic systems have manipulators with arms which are movable to desired locations in accordance with commands in a software program or robotic control mechanism. The arms may contain one or more joints connected by rigid members. Motion of the rigid members about each of the joints may be controlled by a computer or other electronic apparatus so that the end of the robotic arm may be positioned at a desired location.
One of the needs that has developed in connection with greenhouse automation is the need for an effective gripping mechanism, sometimes referred to as an end effector, attached to the end of a robotic arm for securely grasping a seedling so that it may be moved or transplanted to another location. Various kinds of gripping mechanisms have been proposed, but all have proven to be unsatisfactory for the purposes of transplanting seedlings in an automated growing environment. They have proven unreliable in grasping and releasing the seedlings, they have tended to damage or deform the seedlings, and they have not been able to adapt their operation to the varying size and shape of the seedlings. In addition, proposed end effectors, which are attempts to deal effectively with all these problems, are too complicated and expensive.
For example, end effectors which grasp the leaves and stems of the seedlings have been proposed, but these end effectors are not always able to pick up a given seedling and they are subject to the possibility of damaging the plant. See Hwang et al., "A Robotic Pepper Transplanter", Applied Engineering in Agriculture, Vol. 2, No. 1, pp. 2-5, 1986.
End effectors for grasping the root structure of the seedlings have also been proposed. In one example of these arrangements, a pair of parallel flat fingers is inserted into the root structure of the seedling and, to hold the seedling on the end effector, the fingers are moved toward one another while the fingers remain parallel. The seedling is released by moving the fingers apart and withdrawing them from the root structure. This apparatus requires careful alignment and may damage the root structure by compressing it too much. Also, when the seedling is released into a growing flat or pot to complete the transplanting process, the soil in the growing flat or pot is pushed away from the root structure due to the separation of the fingers needed to release the seedling. What may be left in essence is a seedling with crushed root ball in an open hole, when in fact it is more desirable to have the soil in the flat or pot in intimate contact with the seedling around the entirety of a relatively undeformed root ball. See Kutz et al., "Robotic Transplanting of Bedding Plants", Transactions of the ASAE, Vol. 30 No. 3, pp. 586-590, 1987.
Fully instrumented parallel jaw grippers, some with an integrated servo tactile sensing capability, have been developed, but these grippers are too expensive and complicated for commercially viable operations. See Irwin, "An Integrated Servo/Tactile Gripper for Small Parts Assembly", Robots 12 and Vision '88 Conference Proceedings, June 5-9, 1988, Vol. 2, pp. 15:1-15:11.
There has been some patent literature dealing with gripping apparatus for various pierceable articles, such as plants.
Ball et al. U.S. Pat. No. 4,688,837 refers to a robotic end effector which has a pair of outwardly slanted needle gripping devices which grasp and stretch a textile article by moving further outward as they pierce the article. Such an end effector is incapable of grasping an article such as a seedling or similar article.
Disston U.S. Pat. No. 3,602,542 refers to a weeder which has a group of slanted tines to be inserted around a weed to be removed from the ground. The tines are oriented so that a weed may be withdrawn from the ground and separated from as much of the surrounding soil as possible, resulting in an apparatus totally unsuitable for transplanting and like activities. Shredl U.S. Pat. No. 3,596,966 and Fuchs U.S. Pat. No. 3,663,050 refer to other weed removing tools. Behrick U.S. Pat. No. 3,288,513 refers to gaff hook for holding, catching, and retrieving wildlife. Poll U.S. Pat. No. 2,960,944, Armstrong et al. U.S. Pat. No. 4,355,588, Ellis U.S. Pat. No. 4,408,550, deGroot U.S. Pat. No. 4,750,439, and Shaw U.S. Pat. No. 4,788,920 refer to various plant grippers used in planting and transplanting mechanisms which squeeze a large part of a plant between two gripping members. None of these are suitable for automating a transplanting operation because there is a significant chance of plant damage. They also are unable to conveniently accommodate differing size and shape plants without one or more of damage to the plants, an insecure grip on the plants, and a lack of guaranteed release of the plants.
This literature is evidence that over a long period of time many efforts have been made to design a successful gripper mechanism, all involving significant problems. Accordingly, there has been a long felt but unsatisfied need for a gripping apparatus or end effector which avoids the problems outlined above. In satisfaction of this long felt need, Applicants have invented a gripping apparatus which substantially avoids the problems attending the prior gripping apparatus and end effectors.
One object of the invention is to provide a gripping apparatus which is able to reliably pick up an object and reliably release that object.
Another object of the invention is to provide a piercing type gripping apparatus which involves minimal deformation of, and damage to, the gripped object.
An additional object of the invention is to provide a gripping apparatus capable of gripping objects which have different sizes and shapes.
It is another object to provide a particularly suitable apparatus for gripping plants, specifically, the root masses of the plants.
It is yet an additional object of the invention to provide a gripping apparatus which is particularly suited for use in a robotic transplantation apparatus.
It is also an object of the invention to provide a gripping apparatus which substantially avoids the problems of prior gripping apparatus.
Other objects and advantages of the invention are either specifically identified elsewhere in the application or are apparent from that description if not specifically identified.
Applicants have published some information (which does not constitute prior art) about the invention and technology associated with the invention. See (1) Ting et al., "Robot Work Cell Development for Seedling Transplanting", Paper No. 88-1027, presented at the June 26-29, 1988 meeting of the American Society of Agricultural Engineers, (2) Ting et al., "A Robot for Transplanting Plugs", Greenhouse Grower, November 1988, pp. 58-59, and (3) Ting et al., "End-Effector Development for Robotic Transplanting of Seedlings", Paper No. 88-1544, presented at the Dec. 13-16, 1988 meeting of the American Society of Agricultural Engineers. Applicants incorporate by reference the entirety of the disclosure of these three publications into this application.